Ready made formula prep

[Zibidee]

A midwife on post natal ward told us that if you add a small portion of ready made liquid formula to a feeding bottle, and baby finishes this and wants more, you have to use a new feeding bottle to add a second portion, even if you haven't warmed either portion, just at room temp.
I don't understand how a second portion can be contaminated by the first portion of both at room temp, served from the same original bottle of ready made formula.
Is this true?

Always used new - think it’s just something that was said that I didn’t question why

I've literally never heard or done that! And I've only ever used ready made formula to combo feed 2 babies now. As long as you've finished using the bottle 2 hours after the feed began I thought it was fine!